1. Field of the Invention
The present invention relates to a method for installation of a mooring cable or the like for a vessel, preferably for a floating platform structure of the tension moored type, where the mooring cable is connected between the vessel and a point of attachment on the sea floor. The invention also relates to a device for mooring a vessel.
2. The Prior Art
A method of the above type is disclosed in U.S. Pat. No. 3,563,042. In this method a mooring cable is brought out to the installation site rolled up on a reel which is rotatably supported about a horizontal axis on the platform structure. While the platform structure is held in place at the installation site, the mooring cable is rolled off the reel until its lower end reaches down to the sea floor. Here the cable is connected to an already installed attachment point, whereafter the cable is tensioned and secured in the platform structure.
Since tension-moored platforms preferably are used at greater ocean depths, e.g. above 150 m, the mooring cables become correspondingly long. Furthermore, such platforms are usually of substantial size, which again necessitates great dimensions in the mooring cables. The result is that each mooring cable becomes very heavy.
The mooring cables will in use be subjected to very high, partly varying tension loads, and for various reasons it is necessary to give them a very effective protection against corrosion. This is usually done by providing the cables with an outer layer or sleeve or a polymer material, e.g. polyethylene. Even though this protective layer is made relatively thick and is also reinforced, it will nevertheless have a relatively limited resistance against mechanical loads, especially strong, localized external pressure loads.
If a long and course cable of this type was rolled off from a reel as shown in the above-mentioned U.S. patent, the weight of the freely suspended part of the cable would be very high before the lower end of the cable would reach the bottom. This high weight would lead to very high external pressure loads on the cable at the point where it is rolled off the spole. This pressure loading would simply crush the external corrosion protection layer of the cable and thus make this layer ineffective. In addition, such damage would primarily take place in the upper part of the cable which in use would be located in the upper, warmer water layers having high corrosiveness.
Thus, it is the purpose of the invention to provide a method and system of mooring which avoids these drawbacks.